Materials engineering at the nanometer scale can provide smaller devices than those currently available. Recently, one-dimensional (“1-D”) nanostructures, such as wires, rods, belts, and tubes, have become the focal point of research in nanotechnology due to their fascinating properties. These properties are intrinsically associated with low dimensionality and small diameters, which may lead to unique applications in various nanoscale devices. It is generally accepted that 1-D nanostructures provide an excellent test ground for understanding the dependence of physical, electrical, thermal, optical, and mechanical properties on material dimensionality and physical size. In particular, 1-D semiconductor nanostructures, which exhibit different properties as compared with their bulk or thin film counterpart, have shown great potential in future nanoelectronics applications in data storage, computing and sensing devices. Phase-change materials (“PCMs”) are among the most promising media for nonvolatile, re-writable, and highly durable data storage applications. Phase change materials based on the Ge—Sb—Te multi-element alloy system have been extensively studied and have been found to be suitable for optical and electrical memories. Among these alloys, Ge2Sb2Te5 (“GST”) exhibits the best performance when used in a phase change random access nonvolatile memory (PRAM), for speed and stability. GST demonstrates high thermal stability at room temperature, high crystallization rate at high temperatures (can be crystallized in less than 50 nsec by laser heating pulse), and extremely good reversibility between amorphous and crystalline phases (more than 106 cycles). In particular, resistive switching PRAM, using GST thin film as a phase change material, provides faster write/read, improved endurance, and simpler fabrication as compared with the transistor-based nonvolatile memories.
Although substantial improvements in the performance of thin-film-based PRAM have been made over the past decade, a number of issues remain, most notably, large programming current, limited cyclability and scaling problems when moving to increasingly smaller dimensions. There is particularly a concern about the crystalline-to-amorphous phase transition when high current is required for material melting. The Joule heating effect may cause excessive power dissipation and inter-cell thermal interference, presenting problems for further memory scaling.
The phase transition behavior of nanoscale GST may overcome these limitations. The melting temperature Tmelt of GST nanowire (e.g., 385° C. for ≈80 nm diameter) is reduced from that of bulk GST (632° C.), a phenomenon consistent with a previous report for semiconductor nanocrystals that reports a decrease of Tmelt with decreasing nanocrystal size. The thermal conductivity of a low-dimensional structure is also reduced relative to its bulk counterpart. The reduced Tmelt, reduced thermal conductance, and smaller material volume jointly contribute to reduction of the threshold energy required for structural phase transition in GST nanostructures.
A bottom-up synthesis strategy provides a route to prepare various free-standing nanostructures, such as nanodots, nanotubes and nanowires. Some elements or binary compound 1D nanostructures have been synthesized by thermal evaporation, laser-ablation, chemical vapor deposition (“CVD”), and metalorganic chemical vapor deposition (“MOCVD”), etc. methods. One of the challenging issues in the field of PCM nanostructures is thesynthesis of high-quality, high purity Ge—Sb—Te ternary alloy nanowires and nanorods in large quantities.
What is needed is a growth method and associated growth materials for 1-D phase change nanostructures that provide a relatively low melt temperature (e.g., Tmelt≈400° C.) for diameters of the order of 10-100 nanometers (nm), and nanostructures that can easily be grown at pressures close to standard, ambient pressures.